This invention relates to a port transceiver for providing communication between a signal transmitting and receiving machine and a space switch.
In a television production facility, there may be a large number of machines of various types, such as editors and video tape recorders (VTRS). Normally, control messages are distributed among the machines separately from the audio and video information. As shown in FIG. 1, each machine 10A and 10B includes a balanced driver 12 and a differential receiver 14. The balanced driver 12 receives encoded control messages and provides a balanced digital data signal in accordance with the Electronic Industries Association Recommended Standard 422, or RS-422, and the differential receiver 14 receives an RS-422 signal that is decoded into control messages. A female nine-pin D connector 18 at the back panel of the machine has pins 2, 7, 3 and 8 connected to a change-over switch 17. In a first (controlling) state of the machine, the switch 17 connects the two output terminals of the balanced driver to pins 3 and 8 of the connector 18 and connects the two input terminals of the differential receiver to pins 2 and 7 of the connector, whereas in a second (controlled) state the connections are reversed. FIG. 1 shows machine 10A in the controlling state and machine 10B in the controlled state. The two machines are connected together by a cable 20 having male D connectors at its two opposite ends respectively. In this fashion, as shown in FIG. 1, the driver 12 of each machine is connected to the receiver 14 of the other machine. The controlling machine is normally a source machine, i.e. a machine that provides machine control messages in the form of commands, and the controlled machine is a destination machine, i.e. a machine that provides machine control messages in the form of status information. For example, the controlling machine, such as an editor, might instruct the controlled machine, such as a VTR, to enter the recording state, whereupon the controlled machine informs the controlling machine that it is in the recording state, and the controlling machine provides audio and video data to the controlled machine to be recorded thereby.
In order to facilitate connection of a source (controlling) machine to multiple destination (controlled) machines, for example for duplicating from the source machine to the destination machines, each machine may also be provided with a second female D connector 24, as shown in FIG. 2, connected pin-to-pin with the connector 18. The second female D connector 24 is known as connector 0, to distinguish from the connector 18, which is known as connector 1. Connector 1 of the source machine 10A is connected to connector 1 of a first destination machine 10B, connector 0 of the first destination machine is connected to connector 1 of a second destination machine 10C, and so on in daisy chain fashion. Thus, the driver of the source machine is connected to the receivers of each of the destination machines and the drivers of the destination machines are connected to the receiver of the source machine. In effect, the cables and the loop-through connections provide a command bus to which the driver of the controlling machine and the receivers of the controlled machines are connected, and a status bus to which the receiver of the controlling machine and the drivers of the controlled machines are connected. Through a further connection (not shown) the drivers of all the destination machines except the last one in the daisy chain may be disabled, so that only the last destination machine provides status information to the source machine. The drivers that are disabled are placed in a high impedance state, so that their presence does not degrade the characteristics of the signal path from the driver of the last destination machine to the receiver of the source machine.
If cables are used to interconnect the machines, as described above, and the arrangement of the machines is to be changed, the cables have to be swapped and reconnected to establish the new arrangement. This is a time-consuming and error-prone operation.
In order to allow an ensemble of machines to be connected in a flexible fashion in different groups, without need to swap cables whenever a change is desired, the machines may be connected to digital routing switcher 26 having inputs connected to pins 3 and 8 of each female connector and outputs connected to pins 2 and 7 of each female connector, as shown in FIG. 3. In this case, the outputs of each driver are connected to pins 3 and 8 and the inputs of each receiver are connected to pins 2 and 7. In order to connect machine 10A as controlling machine to machines 10B and 10C as controlled machines, for example, crosspoints 30 and 34 are closed to provide the command path and crosspoint 40 or 44 is closed to provide the status path.
A conventional digital routing switcher is composed of row conductors connected to switcher inputs, column conductors connected to switcher outputs, and crosspoint elements for connecting any row conductor with any column conductor. If the crosspoint element for connecting row i with column j is enabled, it asserts a logic high or logic low at output j, depending on the state of input i. If the crosspoint element is not enabled, it does not assert a logic level at output j. However, the crosspoint element does not present a high impedance to output j. Accordingly, the arrangement shown in FIG. 3 is subject to disadvantage because it allows the driver of one machine to be connected through the switcher to the driver of another machine. In the case of the example described above, the driver of the machine 10C is connected through crosspoints 48 and 49 to the driver of machine 10A, even though neither crosspoint 48 nor crosspoint 49 is enabled. Therefore, if the driver of machine 10C outputs a status message, it is delivered not only to the receiver of machine 10A, by way of crosspoint 40, but also to the driver of machine 10A. The resulting signal contention may cause the drivers to cease operation.
A further disadvantage of the arrangement shown in FIG. 3 is that in order to provide full connectivity for Q machines, each having both a connector 1 and a connector 0, 64.times.Q.sup.2 crosspoints are devoted to the interconnections of the drivers and receivers.